In a fully synchronous data transfer system, the interface between the data terminal equipment (DTE) and the data communications equipment (DCE), such as a modem, is synchronous and the interface between the DCE's is also synchronous. In this type of an environment, the modems operate as simple data transfer devices. That is, the DTE's control the direction of the data transfer, are responsible for error detection and control, data compression/decompression, and/or data encryption/decryption. The modems therefore function as "dumb" devices whose sole function is to transfer data between the DTE and the data link (e.g., a telephone line).
In half duplex operation, the modems take turns transmitting. The length of an individual transmission is dependent upon the amount of data that the DTE has to send and any packet size limitation imposed by the protocol being used. Also, in half duplex operation, since only one modem can transmit at a time, then during the period when a first modem is transmitting the receiver for that modem is not receiving a signal from the second modem. As a consequence, the receiver clocks and phase-locked loops for the first modem gradually lose synchronization with the transmitter clocks in the second modem and require resynchronization before data can be successfully transferred. Therefore, at the beginning of each transmission, a retraining sequence is sent. The retraining sequence allows the receiver clocks of one modem to resynchronize to the transmitter clocks of the other modem. This retraining sequence typically has a fixed duration which is sufficient to cause resynchronization for short burst, half duplex operations.
However, the amount of resynchronization actually required is an approximately linear function of the period for which the receiver was receiving no data. Therefore, if the receiver has not received a signal from the other transmitter for a short period, then only a short retraining sequence is necessary to achieve resynchronization. It will be appreciated that prefixing a fixed duration retraining sequence to every transmission can significantly reduce the effective data transfer rate when the data is transferred in short bursts. Likewise, if the receiver has not received a signal from the other transmitter for a long period, then a longer retraining sequence is necessary to achieve resynchronization. It will be appreciated that this fixed duration retraining sequence may be insufficient to achieve resynchronization when there is a long period between line turnarounds. However, existing modems provide only for a fixed length retraining sequence, regardless of the amount of time between transmissions. Therefore, there is a need for a modem which provides a retraining sequence which has a duration dependent upon the period of time that the opposing receiver has not been receiving a signal. Also, there is a need for a modem which expects a retraining sequence which has a duration dependent upon the period of time that the opposing transmitter has not been transmitting a signal.
Furthermore, because of transmission link delays and different clock frequency tolerances, the two modems will not necessarily agree upon the amount of time that a particular receiver has not been receiving a signal. This can cause data loss if the duration of the retraining sequence is insufficient to resynchronize the receiver or if the receiver is still expecting additional retraining even after the opposing transmitter has begun transmitting data. Therefore, there is a need for a modem which provides a retraining sequence which has a duration dependent upon the amount of time since the last transmission and which compensates for delays in the data transmission link (e.g., a telephone line). Likewise, there is a need for a modem which expects a retraining sequence which has a duration dependent upon the amount of time since the last reception and which compensates for delays in the data transmission link.
The provision of an adjustable duration retraining sequence has two effects: higher data transfer rates; and incompatibility with previously existing modems. The first effect is desirable. The second effect is an undesirable consequence. If the modem is to have maximum utility, the modem should provide for both fixed duration and adjustable duration retraining sequences. The modem can then maintain compatibility with existing equipment and also provide a higher data transfer rate when it is communicating with another modem which also has the adjustable duration retraining sequence feature. However, the modem must have a way of determining whether the other modem is a conventional modem or is an improved modem with the adjustable duration retraining sequence feature. Therefore, there is a need for a handshaking protocol which will allow a first modem to determine whether a second, connected modem is of a conventional design or of an improved design.
It is a simple matter to provide for a handshake signal. However, it is substantially more difficult to provide for a handshaking sequence which can be used in conjunction with existing handshaking sequences without disrupting the operation of these existing handshaking sequences. CCITT Recommendation V.22 et al. provide for a handshaking sequence so that a modem can determine whether the modem to which it is connected operates at 300, 1200 or 2400 bps. In addition, U.S. patent application Ser. No. 054,419, filed May 26, 1987, by Tjahjadi et al., entitled "High Speed Half Duplex Modem With Fast Turnaround Protocol" and assigned to the assignee of the present invention, hereby incorporated herein by reference, describes a handshaking sequence which, in addition to maintaining compatibility with the standard 300/1200/2400 bps handshaking sequence, also provides for a selection of 4800 or 9600 bps operation. Therefore, there is a need for an additional handshake sequence which is compatible with existing handshake sequences and further provides an indication that the modem is of an improved type.
There is currently no feature negotiation capability in synchronous modems, so the addition of any new feature requires that a new or modified handshake be provided.
With the expanding number of present and possible enhancements or features that are currently available and/or may become available in the near future, it is desirable to have a handshaking sequence which can provide for negotiation of a plurality of different features between the two modems. The handshaking sequence should accommodate the negotiation of any existing features and also provide for negotiation of additional features which may be provided in the future. Therefore, there is a need for a handshaking sequence which is compatible with existing handshake sequences and also has the capability to allow negotiation of a plurality of features or enhancements.